This TS provides an overview and overall description of the E-UTRAN radio interface protocol architecture. Details of the radio interface protocols are specified in companion specifications of the 36 series.

The E-UTRAN consists of eNBs, providing the E-UTRA user plane (PDCP / RLC / MAC / PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface.

The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1‑MME and to the Serving Gateway (S-GW) by means of the S1‑U.
The S1 interface supports a many-to-many relation between MMEs / Serving Gateways and eNBs.